Adjustable current sources are used in a variety of applications. An example of one application is the use in squib drives for automotive airbags. A reason for using the adjustable current source is the variable resistance of the squib and other application specific requirements. However, the adjustable current flowing through the squib is also used to generate a supply voltage for other electronic circuits. A simplified circuit diagram of a typical application of an adjustable current driver is shown in FIG. 1.
Referring to FIG. 1, the reference numeral 100 generally designates the conventional system. In this illustrative example, the magnitude of the adjustable current IADJ can be varied between three different target values I1, I2, and I3. The target values may range from 10 mA to 100 mA due to production spread of the resistance of squib Rx and other requirements of the application. In this example, the resistance of squib Rx may, for example, vary between about 1Ω and about 6Ω. The adjustable current source 102 is typically controlled by a digital control signal CNTL0. After the current IADJ has passed through the squib Rx, it is used to generate a substantially constant output supply voltage VOUT at the output node or pin 114. Output voltage VOUT is used as a voltage supply for diagnostic circuitry 108, which can determine, for example, the state of the squib Rx and other components. Output voltage VOUT can also be used to supply further signal processing circuitry 112. In order to maintain the output voltage VOUT at a rather constant level, a Zener diode Dz is used that should provide a desired voltage level for the output voltage VOUT.
One problem with the system 100 is that the pins 104 and 106 can be shorted to a supply voltage level. A short circuit between pins 104 and 106 can result in large currents flowing into circuit 100, potentially damaging or destroying various circuits. Therefore, a current limiter 110 is provided to limit the amount of current that can pass through the Zener diode Dz and flow into the circuit 116. In a typical application, the current is limited to twice the target value (i.e., for a target value of 10 mA, the current limit is set to 20 mA and for a target value of 100 mA, the limit will be set to 200 mA). However, a Zener diode Dz, which is integrated on a semiconductor device, occupies a substantial amount chip area in order to be able to withstand currents as high as several milliamps. Additionally, if a current exceeds the current limit, the output voltage VOUT at pin 114 would rise regardless of the breakdown voltage of the Zener diode.
Some other examples of conventional systems are European Patent No. EP1769974; U.S. Pat. No. 7,142,407; U.S. Pat. No. 7,154,733; and U.S. Patent Pre-Grant Publ. No, 2005/0225924.